Perhaps it was the concern that everything on the series - from the bridge to the uniforms - had been designed for television screens and not cinema screens and thus might show up on cinema screens in a negative light?
Much has been made of how this was the official reason for the redesign of the bridge and the director of photography's lighting of the film. Perhaps the uniform switch (which admittedly in the end turned out to be rather futile) was merely to avoid having people notice coffee stains and fabric runs on the big screen?
Ah, but weren't the DS9 uniforms designed for TV? How is switching one set of TV uniforms for another any better?